Add action for containerbuilds

[eddelbuettel]

This PR adds a simple-enough workflow to create containers used for CI as well two convenience add-ons.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Prefereably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[Enchufa2]

LGTM. Note that, if you wish, it could be further simplified by stacking together the build&push steps for run and plus in the same job, just one after the other:

    - name: Build and push
      uses: docker/build-push-action@v2
      with:
        push: true
        context: docker/run
        tags: rcpp/run

    - name: Build and push
      uses: docker/build-push-action@v2
      with:
        push: true
        context: docker/plus
        tags: rcpp/plus

[eddelbuettel]

Fair point. I will consider that for another revision.